In the proportioning of a miscible solute in a water solvent under continuous flow, objectionable ratio variation of over 50% can occur in the simple low cost conventional systems because of pressure variations in either the solvent supply and/or the solute supply lift heights. The conventional practice particularly with portable units has predominantly become the use of either one of two designs in single stage related to the relative pressure of the solute at the mixing chamber, both having their advantages and disadvantages:
(1) High Vacuum, metered solute flow: A mixing system which develops a deep vacuum condition approaching zero p.s.i. absolute upon the solute, for conducting the solute through a flow restriction at the mixing chamber level in order to meter the amount of solute desired for a ratio proportioning related to a resulting high vacuum effect thereof in the mixing chamber in order to reduce the significance of lift height variations on the solute supply pressure to the flow restriction; or,
(2 ) Low Vacuum, free solute flow: A mixing system in which the pressure acting upon the solute in the confluence chamber essentially differs only by the free flow low vacuum lift height variations of the solute and proportioning is essentially independent of variations in solvent pressure.
In the high vacuum metered flow system, since vacuum is related to the pressure of the environment, the ratio proportioning in the first stage is adversely affected by variations in solvent pressures, and, at lower ratios because of the extensive deterioration occasioned by the high vacuum variations on the solute supply to the confluence chamber. In the low vacuum free flow system, the ratio proportioning is adversely affected by variations in solute lift heights, particularly at higher ratios because of the increase of ratio degradation with increasing ratio and also at higher ratios there is greater difficulty of providing and maintaining the relationship between inlet and outlet ports of the mixing chamber.
Although either system is quite accurate if the exact design solute and solvent pressures are used and their relationship remains constant, only a small percentage of the commercial market would be satisfied and the danger of improper solution ratios still confront most of the users since standard concentrates are supplied and solvent pressures available significantly vary quite widely.
Thus, at conventionally tolerated performance ratios which are available, the solute metered flow system is generally used as limited to ratios above a 1 to 10 and the free flow solute system limited to ratios below 1 to 24. Two-stage conventional high vacuum gap proportioners may provide a better pressure output efficiency, but they perform no better than single stage proportioners with respect to ratio variations caused by the solvent pressure variations occurring in both stages.